Jack H. Judy

Oxygen as a surfactant in the growth of giant magnetoresistance spin valves

We have found a novel method for increasing the giant magnetoresistance (GMR) of Co/Cu spin valves with the use of oxygen. Surprisingly, spin valves with the largest GMR are not produced in the best vacuum. Introducing 5×10−9 Torr (7×10−7 Pa) into our ultrahigh vacuum deposition chamber during spin-valve growth increases the GMR, decreases the ferromagnetic coupling between magnetic layers, and decreases the sheet resistance of the spin valves. It appears that the oxygen may act as a surfactant during film growth to suppress defects and to create a surface which scatters electrons more specularly. Using this technique, bottom spin valves and symmetric spin valves with GMR values of 19.0% and 24.8%, respectively, have been produced. These are the largest values ever reported for such structures.

Composite media (dynamic tilted media) for magnetic recording

We designed and fabricated a composite magnetic recording medium with exchange decoupled magnetic grains that consist of two vertically exchange-coupled magnetic regions (one is magnetically soft and one is magnetically hard) as an approach to alleviate the writing field limitation of perpendicular magnetic recording heads. A nonmagnetic layer with different thickness was put between the hard and soft layer to tune the exchange coupling. With proper coupling, significant drop of the coercivity field was observed for this composite medium while still maintaining good thermal stability. Better recording performance was obtained for such medium compared to perpendicular and longitudinal medium. The results have proved the possibility of fabricating a writable recording medium having an ultrahigh magnetic anisotropy constant (Ku) value.

Magnetoresistance values exceeding 21% in symmetric spin valves

We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR.

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.

Specular electron scattering in giant magnetoresistance spin valves

Specular scattering of electrons at the top surface of bottom spin valves has been found to contribute to the giant magnetoresistance effect. The specular scattering can be increased by the deposition of /spl sim/2 monolayers (ML) of Au, Ag, or Cu, with the result that the GMR increases and the R/sub /spl square// decreases. The deposition of 2 ML of species that disorder the surface, such as Ta, Si, C, or Ni/sub 80/Fe/sub 20/, suppresses the specular scattering. This suppression appears to be useful as a new method for evaluating the importance of specular scattering.

Effects of platinum content and substrate bias on the structure and magnetic properties of CoCrPt/Cr thin films

The effects of the Pt content (0–40 at. %) and the developed dc substrate bias voltage (0 to −190 V) on the magnetic properties and structure of rf sputtered CoCrPt/Cr films with two different remanent magnetization‐thickness products Mrδ (0.7 and 2.3 memu/cm2) were studied. It was demonstrated that a wide range of in‐plane coercivities (500–3450 Oe) can be easily obtained in these films. The addition of Pt affected the magnetic properties of the films through changes of the lattice parameter, texture, and phase composition. A maximum in the in‐plane coercivity exists for moderate amounts of Pt. The saturation magnetization decreased monotonically with increasing Pt content. rf substrate bias increased the in‐plane coercivity and altered the character of the magnetic interactions through changes of the stress, texture and microstructure of the magnetic layer.

Durability and structure of RF sputtered carbon-nitrogen thin film overcoats on rigid disks of magnetic thin film media

Thin carbon film overcoats were deposited on rigid disks of magnetic thin film media by RFD diode sputtering using a mixture of argon and nitrogen gas. Constant drag test (CDT) and initial take-off velocity of thin film head sliders were measured using a computer-controlled friction-wear tester. As the nitrogen partial pressure was increased from 0 mTorr to 7.5 mTorr, the number of revolutions before wear failure increased from 3000 to 9000 revolutions and the initial take-off velocity decreased from 3.82 m/s to 2.58 m/s. The chemical bonding and elemental composition as determined by X-ray photoelectron spectroscopy and the microstructure as observed by transmission electron microscopy were used to correlate the durability and structure of RF sputtered carbon-nitrogen thin films. >

NIFE/NIO BILAYERS WITH HIGH EXCHANGE COUPLING AND LOW COERCIVE FIELDS

NiO/Ni81Fe19 bilayers with high exchange coupling, Hex, and low coercivity, Hc, were fabricated. These bilayers with NiO crystalline textures of (111), (200), and (220), and different surface/interface roughness were deposited onto a Si(100) wafer by changing the rf reactive sputtering conditions. A new parameter of exchange coupling to coercivity ratio, Hex/Hc, was used to quantify the effectiveness of the exchange bias for the bilayer film. An Hex=39 Oe and Hc=4.1 Oe were obtained from a NiO (45 nm)/NiFe (10 nm) bilayer, yielding a ratio of Hex/Hc=9.51, which was significantly higher than any previously reported results. An atomic force microscopy study showed that the key to the low coercivity was the reduction of surface roughness of the NiO/NiFe bilayer. It is proposed that the roughness-induced magnetostatic coupling in the NiFe film could be one of the main mechanisms for the relating high coercivity in NiO exchange-biased NiFe bilayer. NiO/NiFe/Co/Cu/Co/NiFe spin-valve films were deposited and a g...

Growth of giant magnetoresistance spin valves using Pb and Au as surfactants

We have investigated the use of Pb and Au as surfactants in an attempt to achieve smoother and sharper interfaces in three types of giant magnetoresistance (GMR) spin valve multilayers: symmetric spin valves, bottom spin valves, and top spin valves. The coupling fields are reduced by a factor of 10 for symmetric and bottom spin valves and by a factor of 3 for top spin valves, presumably by suppressing roughness and interdiffusion at the Co/Cu/Co interfaces, when ∼1 monolayer of Pb is deposited in the early stages of spin valve growth. The Pb has a strong tendency to float out or segregate to the surface during deposition of the spin valve leaving the GMR largely unaltered. Au is almost as effective as Pb, however the Au tends to be left behind in the spin valve, and the GMR is reduced slightly. Attempts to use Hg as a surfactant were unsuccessful. The coupling field increased, and the GMR decreased sharply.

Co-Cr films with perpendicular magnetic anisotropy

The magnetic properties of RF sputtered Co-Cr films have been studied over a range of preparation conditions extending previous work. Reproducible well-developed perpendicular anisotropy, as would be required for perpendicular recording, was found when the deposition conditions were maintained at an optimum of 170°C substrate temperature, 1.5 KV RF voltage and 5 microns argon pressure. Film thickness was nominally 1 micron. The optimum deposition rate in our system was 120 angstroms/minute due to the differential sputtering rates of Co and Cr. The resulting films had coercivities up to 1500 oersteds and (00.2) hcp X-ray peak width ΔΘ 50 of 5 degrees, with secondary X-ray peaks observed as well. Columnar growth was observed with grains less than 800 angstrom diameter. A sublayer of poor orientation was observed. Magnetic moment was observed to be about 35% higher than that reported by Iwasaki. Initial background pressure less than 6 × 10-7torr was essential for formation of well-oriented films. The presence of nitrogen caused the appearance of the fcc phase and the loss of perpendicular coercivity. Loss of coercivity also occurred when film composition was nonuniform.